1. Field of the Invention
The present invention relates to a process for recovering N-methylpyrrolidone (hereinafter referred to as "NMP") in the preparation of a polyarylene thioether (hereinafter referred to as "PATE") by using NMP as the polymerization solvent.
More particularly, the present invention relates to a process for the recovery of NMP in which NMP left in the residue of a distillation column for the recovery of NMP in the post treatment process after termination of the polymerization reaction is recovered; the risk of ignition of the residue in the step of discharging the same from the distillation column is eliminated; generation of an unpleasant smell at the manufacturing site is prevented; and the NMP thus recovered is economically utilized again.
2. Prior Art
NMP is a typical heat-resistant organic polar solvent and has recently been valuably used as a polymerization solvent, especially for the production of PATE. However, since NMP is expensive, it is an indispensable condition for the industrial production of PATE from the economical viewpoint that NMP left in the reaction mixture liquid be recovered in a high yield after the polymerization reaction and that the recovered NMP be recycled to the polymerization process.
As a conventional method for recovering NMP contained in the reaction mixture liquid after the polymerization reaction process, a process in which a high-temperature reaction mixture liquid is subjected to adiabatic flashing to evaporate NMP and the NMP gas produced is cooled, condensed and recovered has been adopted (see, for example, U.S. Pat. No. 3,941,664 and U.S. Pat. No. 3,956,060 and Japanese Patent Application Laid-Open Specification No. 53324/86). According to this process, however, the recovery is low because the amount of NMP which is not evaporated but is left is considerably large, whereby the process is unsatisfactory from the industrial viewpoint.
As a measure for eliminating this disadvantage, a process in which NMP-containing liquids formed at various steps of the post-treatment process are finally collected irrespectively of whether or not adiabatic flashing is carried out, and the collected liquid is subjected to distillation by using a distillation column or the like for recovery of NMP has been proposed (see, for example, the specification of U.S. Pat. No. 3,783,138). However, since considerable amounts of solids such as oligomers and salts are contained in the collected NMP-containing liquid, the residue in the distillation apparatus (hereinafter referred to as "distillation residue") will lose flowability upon excessive distillation of NMP, and it will become difficult to discharge the distillation residue from the distillation apparatus.
Furthermore, if the distillation temperature is elevated to amply distill NMP, the contained organic substances such as NMP and PATE oligomers will become decomposed, and impurities having a higher boiling point than that of NMP will become mixed into NMP recovered and are apt to have adverse effects on circulation and re-use of NMP.
Accordingly, in the conventional process, the recovery by distillation of discharged NMP can be performed only to such an extent that the distillation residue will still be in the form of a sufficiently flowable slurry.
A large amount of NMP is thus still left in this slurry of the distillation residue, but according to the conventional technique, this NMP is burnt or discarded. This distillation residue slurry has a high vapor pressure of NMP because the temperature is high, whereby there have been environmental problems such as the risk of ignition, unpleasant smell of the slurry, and inhalation of gas by workers. Moreover, the loss of NMP in the recovery process results in increase of the manufacturing cost of PATE.